The pervasive quest for ever-increasing data capacity for an ultra-small form factor hard drive (e.g. sub-7 mm) is conventionally limited to a single disk configuration. In order to accommodate an additional disk such as to increase data capacity within the same form factor height or vertical confinement (Z-height), it would require (i) physically lowering of the disk seating surface and (ii) additional electromagnetic force to shoulder the increased rotating load.
For the former, conventional axial field spindle designs usually require the coil or magnet polarity to be contained within the inner rotor hub periphery. The accommodation of another disk media within the same form factor height entails increasing height spacing (disk headroom) either on stator or rotor dimensions. However, this is not possible in a conventional axial field spindle motor due to the presence of the stator directly under the disk seating surface which would block the disk seating surface from being lowered. For ease of understanding, FIG. 1A illustrates a conventional axial field spindle motor 100 for a small form factor HDD and FIG. 1B illustrates a conventional deposited-on-coil substrate (stator) 110. From FIG. 1A, it can be seen that the disk seating surface 102 cannot be lowered in order to accommodate an additional disk due to the presence of an outer part of the stator 110 being in the way. In particular, the conventional stator 110 has a tab extension 120 protruding from the circular periphery (outer circumference) of the stator 110 on which contact pads for connecting with power lines are formed as shown in FIG. 1B. This tab extension 120 increases the span of the stator 120 in the radial direction and thus presents problems for the conventional axial field spindle 100 to accommodate additional disk(s) as explained above.
For the latter, the added load from the additional disk(s) and spacer(s) leads to a higher torque being required to rotate the entire assembly, thereby warranting more coil layers or turns but would be limited by magnetic flux leakage due to air gap penalty.